You’re right, the TouchInjection API is unique to Windows 8.
For Windows 7 you have to go through a lot of pain to generate WM_TOUCH events, because only drivers can do this. And writing a driver for Windows 7 is no walk in the park.
You might want to take a look at the source code of MultitouchVista: http://multitouchvista.codeplex.com
But if you want to take my advice: Don’t. Windows 8 will be available within weeks (if you don’t already have access), it’s a LOT faster than Windows 7 and is designed for multitouch. Then you could use the TouchInjection API yourself. (Or wait until I finished my TUIO->TouchInjection app )